This invention relates generally to converting natural gas components into higher molecular weight hydrocarbons; more particularly it concerns an improved one-step process for such conversion, and employing methane chlorination.
In many areas of the world there are abundant supplies of natural gas which are widely distributed by pipeline systems. The composition of natural gas varies with the source but essentially it is made up of methane, (typically about 80% by weight), ethane, propane and other paraffinic hydrocarbons, along with small amounts of inorganic gases. Natural gas is mainly fuel.
For chemical stock consumption, current practices are mainly limited to the production of ammonia, hydrogen, methanol and carbon black. Natural gas has received limited use as a chemical feedstock principally because methane is the most stable of all hydrocarbons. To convert methane directly to other hydrocarbons involves both thermodynamic and kinetic barriers. For example, both net changes of enthalpy and Gibbs energy of the following reaction are unfavorable at all temperatures: EQU CH.sub.4 .fwdarw.1/nC.sub.n H.sub.2n+2 +(1-1/n)H.sub.2 ( 1)
Production of olefins and acetylenes involves even less favorable enthalpies of reaction. Their Gibbs energies of reaction become favorable only at temperatures above 1200.degree. K. However, at these high temperatures, the route for the production of carbon and hydrogen is more favorable. Since methane is the most stable and has the strongest C--H bonds, any free radical process capable of producing "polymers" would tend to attack these polymer products more readily than to attack the parent methane. This process limits the yields attainable, kinetically.
Conversion of methane to ethylene, acetylene and hydrogen by using chlorine gas as a catalyst or a coreactant has previously been described. For example, Gorin et al taught in U.S. Pat. No. 2,488,083 that methane can first be converted to methyl halide and then catalytically condensed to hydrocarbons having two or more carbon atoms to the molecule. Benson disclosed in U.S. Pat. No. 4,199,533 that ethane, ethylene and hydrogen can be produced from methane by reacting methane with chlorine at 700.degree. C. or higher. U.S. Pat. No. 2,320,274 to Gorin describes production of benzene, ethylene and acetylene from two step reaction of methane and chloride, at temperatures between 600.degree. and 1000.degree. C., and residence times in excess of 10 seconds. If methane can be readily converted to basic petrochemical building blocks such as ethylene and acetylene, the use of natural gas as chemical feedstock is enhanced.